1. Field of the Invention
The present invention relates generally to an orthogonal frequency division multiplexing (OFDM) communication system, and in particular, to a method and apparatus for allocating pilot carriers adaptively to implement orthogonal frequency division multiple access (OFDMA).
2. Description of the Related Art
A typical OFDMA communication system transmits parallel transmission data in mutually orthogonal Nused carrier frequencies. Nguard carriers having null data are inserted as guard intervals at both sides of the total frequency band. Hence, a total of NFFT (=Nused+Nguard) carriers carry the transmission data. The Nused carriers contain N pilot carriers and (Nused−Npilot) data carriers. Both a transmitter and a receiver know the pilot carriers. Traffic channels are estimated from the phase distortions of the pilot carriers. If the pilot carrier phase distortion is small, channel estimation is carried out with a small number of pilot carriers. Alternatively, with great pilot carrier phase distortion, more pilot carriers are required for acceptable channel estimation performance.
In OFDMA, the Nused carriers are divided into Nsubch sub-channels. One sub-channel is a unit in which a subscriber transmits data. In other words, the subscriber transmits data on one or more sub-channels. Each sub-channel contains Nsubcarrier sub-carriers. If each subscriber is connected to one sub-channel in OFDMA, the sub-channels are available to up to N (=Nsubch) subscribers. As a result, the number of subscribers can be increased.
With this advantage, OFDM wireless communication systems for high-rate data transmission have been developed, such as digital audio broadcasting (DAB), digital video broadcasting (DVB), digital terrestrial television broadcasting (DTTB), wireless local area network (LAN), and wireless asynchronous transfer mode (ATM). Needless to say, techniques of allocating pilot carriers to each subscriber are essential to implementation of these OFDMA communication systems.
Pilot sub-carrier allocation is also significant to digital wired communication systems adopting discrete multi-tone (DMT) such as asymmetric digital subscriber line (ADSL) and very-high bit rate digital subscriber line (VDSL).
FIG. 1 illustrates a typical OFDMA symbol structure in the frequency domain. Referring to FIG. 1, three sub-channels 101, 102, and 103 are defined. As stated above, one sub-channel is a unit to which one subscriber is connected and Nsub sub-channels contain data carriers and pilot carriers. Guard intervals 104 attenuate a signal smoothly in the waveform of a brick wall, thereby preventing interference to adjacent connected channels. A DC sub-carrier 105 represents the center of the frequency band.
Referring to FIG. 1, the total channel frequency is logically divided into sub-channel 1 101, sub-channel 2 102, and sub-channel 3 103 to accommodate as many subscribers as possible by multiple access. Each sub-channel contains different carriers.
FIG. 2 illustrates a conventional carrier allocation method depicting how carriers make up an OFDMA sub-channel. To form a sub-channel with carriers, a particular formula is used. The positions of variable-position pilot carriers 201 are first decided and then the positions of data carriers 202 are decided according to the formula. A constant-position pilot carrier 203 is inserted at a predetermined position between a variable-position pilot carrier 201 and a data carrier 202
Referring to FIG. 2, for transmission of OFDMA symbol 1, the variable-position pilot carriers 201 are selected. Reference numeral 205 denotes indexes indicating the positions of carriers for OFDMA symbol 1. In practice, Nsubch—data data carriers and NSubch—pilot pilot carriers are selected among Nused usable carriers across a predetermined frequency band, for forming a sub-channel. Here, Nsubch—pilot results from dividing the number of pilot carriers by the number of sub-channels. In other words, the sub-channel for a user contains (Nsubch—data and Nsubch—pilot) carriers across the total frequency band. In this manner, the variable-position pilot carriers 201 are positioned at indexes 0, 13, 27, and 40, the constant-position pilot carrier 203 at index 26, and the data carriers 202 at the remaining indexes for OFDMA symbol 1. If subscribers transmit OFDMA symbol 1, they do so using different carriers in different sub-channels having pilot carriers at the same logical positions. Since the sub-channel forming formula is a function of the number of subscribers, each subscriber is assigned a sub-channel having different carriers.
Since OFDMA subscribers transmit data on sub-channels irrespective of NFFT, the numbers of data carriers and pilot carriers are divided by the number of the subscribers, for carrier allocation. Here, the number of pilot carriers is fixed. This implies that there is no problem when a sub-channel is in a good state, but the phase error of a traffic channel is not accurately estimated when the sub-channel in a bad state, thereby degrading channel estimation performance.